Electric immersion heater



Get. 8, 1940 J D MORGAN ET AL 2,217,595

-ELECTRIC IMMERSION HEATER Filed Feb. 13, 1939 IN EN P E R C sf L 3 1 v3 ATTORNEY Patented Oct. 8, 1940 PATENT OFFICE ELECTRIC IMJVIERSIONHEATER John D. Morgan, South Orange, and Percy B. Levitt, Millburn, N.3., assignors to Power Patents Company, Hillside, N. 1., a corporationof Maine Application February 13, 1939, Serial No. 256,020

2 Claims.

The present invention relates to electrical heating devices, and moreparticularly to an electrical heater adapted for immersion in liq-- uidfor the purpose of heating the same.

A particular object of the invention is to pro vide an electricimmersion heater which is adapted for heating the lubricating oil supplyof an airplane to a. predetermined maximum temperature.

With the aforesaid object in view, the present electrical heater hasbeen designed to meet therequirement of an electric heater which shallbe compact in size, light in weight, eiilcient in developing andtransferring heat to the liquid in II which it is immersed, and ofsuchconstructlon as to withstand, without injury to its parts, thevibrational and other shocks and jars to which it may be subjected whilein use in an airplane oil tank.

3 With the above and other objects and few tures in view, the inventioncomprises the 1m proved immersion heater which is hereinafter describedand more particularly defined by the accompanying claims.

8 The various features of the invention are illustrated by theaccompanying drawing, in which:

Fig. 1 is a longitudinal sectional view of a preferred design of heaterimmersed in a receptacle of oil or other liquid to be heated.

Fig. 2 is a cross sectional view of the heater taken on the line 2-4 ofFig. 1;

Fig. 3 is a cross-sectional view of the heater taken on the line 3-3 ofFig. l;

Fig. 4 is an enlarged longitudinal sectional view of a small portion ofthe heater illustrating the dielectric coatings for the resistanceelement.

Referring to the drawing, Fig. 1 illustrates w a heater Ill of thepreferred type submerged in oil or other liquid in a tank It havingwalls It. Since one requisite of the heater is mechanical strength towithstand shocks, it is provided with a strong dielectric refractorycore it which pref- 5 erably has the form of a cylindrical tube with acentral longitudinal bore It. A preferred refractory material for theconstruction of this core is a mixture of about equal parts by weight ofnatural and milleagratn zircon which has been heat bonded by thereaction products supplied by about 6% of the weight of the zircon oforthophosphoric acid.

The heating element consists of a helical coil of wire I! wrapped withmany turns about the periphery of core I4 throughout its length. In

order to provide a compact heating device of high heating efficiency,the element is pref erably made of small diameter high temperatureresistant alloy having closely spaced turns. A preferred wire is one ofabout #22#26 gauge 8- made of an alloy of 30-40% chromium, l-8%aluminum, 0 5% cobalt and the balance iron. In order to obtain closespacing of the turns of wire along the length of the core-without shortcircuiting the wire may be protected before it) wrapping it around thecore in constructing the heater, by applying a high voltage heatingcurrent thereto in an air or other oxidizing atmosphere, whereby a metaloxide film 26! (Fig. at is built up on the surface of the wirethroughout its length. As an additional precaution against shortcircuiting of current between in dividual turns of the wire coil orhetweeri wire and a metal casing 22 of the heater a low temperature 11dial coating M is built up on 5 Us. the wire wrapped core and hardenedby halting thereon, as a step in the construction. of the heater. Thethickness of coating 24 around. the wire coil is preferably onlysufficient to insure 9& against shorting to the casing 22. A preferredmaterial for the coating is a mixture of milled grain (under 300 mesh)zircon and 30% natural grain (thru 40 mesh) zircon bonded with 6% P205and 2% chromic acid. This coat- 30 ing can be hardened by baking at atemperature as low as 600 F.

After wrapping the oxide coated wire ll around the core H and applyingand baking the dielectric coating 24. the ends of the coil of wire 85 I8are connected to lead-in wires 28, and the resulting unit is insertedwithin the metal casing 2!. Wires 26 are shown extending to a thermostatswitch" on the outside of tank 92 through a tubular extension 30 ofcasing 22. The wires 40 28 are mounted within casing 30 in protectivedielectric tubes 32 of porcelain or other suitable refractory; tubes 32being rigidly centered within the casing by a heat bonded dielectricrefractory filling 34 which physically bands the 4;; tubes 32 and casing30 together. The rei'rae tory shell 34 may preferably be made of thesame refractory material employed in the coating 2|, and should beJoined integrally with the coating 24 at the heater end of the tube 30,as 5s illustrated.

After inserting the wire wrapped and coated core in the casing 22 in theposition illustrated, the next step in the construction of the heater isto fill the annular space between the inside 55 -of the tank ll.

surface of the casing 22 and the outer surface .gof the coating 24 witha highly heat conductant compactly tamped dielectric powder filling 36.A preferred material for the filling 36 is prepared by first heatingfinely divided (thru 150 mesh) ferrosilicon powder silicon) in anoxidizing atmosphere to a temperature of 2350" F. while agitating thepowder to build up an oxide coating or film on the individual grains.The purpose of thus imparting an oxide coating to the particles is toincrease the dielectric properties of the powder-and accordingly thisstep may be omitted in c ase the coating 24 is thick enough to insureagainst shorting of current between the resistance coil and casing 22.The powdered ferrosilicon is compactly tamped in the space betweencasing 22 and coating 24-as by vibrating the unit while introducing thepowder. Before sealing the heating unit by welding an end plate 36 tothe casing 22, electric current is preferably applied to the wire coilill in amount sufiicient to thoroughly heat and dry out all partsthereof, including the packed powder filling 36. The sealed casing 22,together with the powder filling 66, is designed to afford a highly heatconductant, corrosion and leak age-proof housing for the resistancecoil, with suflieient strength and flexibility to protect all parts ofthe heater against breakage and injury.

The heating unit is supported within the tank H by being clamped bymeans of a nut 40 .to a metal cap 42 having threads on its peripherywhich engage threads in a metal ring 44 which is in turn inserted in acircular aperture in one of the tank walls I2 and may be welded inplace. A circular metal plate 46 is tightly fitted in an annular recessformed by shoulders 46 on rearwardly projecting extensions of cap 42;plate 46 forming the base of a housing for the switch 26. The leads 26are connected through the switch with a cable 50 leading from a sourceof electric current.

An open ended tubular metal well 52 has its side walls at the open endwelded'or otherwise connected by a sealed joint. to the plate 42. Theclosed end of the well 52 extends into an annular chamber 54 lyingbetween the outer wall of extension 30 of casing 22 and the inner wallof an open ended metal shell 56. The rearward end of shell 56 isthreaded to engage threads on an annular forwardly projecting shoulder56 of cap 42. One or more apertures 60 are provided in the rearward sidewalls of shell 56, communicably connecting chamber with the interior Aplurality of webs 62 extend inward radially from the forward inner wallof shell 56 to form a plurality of longitudinal groove passages 64 forcirculation of liquid in a plurality of streams from one end to theother of shell 56 in intimate heat transfer contact with heater casing22. This construction-permits rapid circulation of the liquid over theheater surface without formation of eddy currents.

The switch 28 is designed for operation as a thermostat switch wherebyto limit the maximum temperature to which the liquid in the tank I l isheated. The mechanism for automatically opening the switch to cut oilthe supply of current to the heater when the liquid has reached apredetermined temperature may be broadly des ignated as a vapor tensionSylphon. The essential elements of this mechanism consist of a smallvapor expansion chamber 66 formed on the switch side of an aperture 68in plate 46 and lying between the plate and a flexible spring metal discdiaphragm HI; together with a liquid filled metal capsule 12 mounted inwell 52 with wardly against an adjustably hinged leaf spring 76, one endof which is hinged to a shoulder of plate 46 by a screw I6. The otherend of spring 16 has an aperture or slot through which passes a bolt 60,one end of which is aflixed to the plate 46, while the other end carriesa nut 62 which serves as an adjustable shoulder for one end of a coilspring 64. The other end of spring 64 holds leaf spring 16 underadjustable compression out of contact with an actuating pin 66 for theswitch 26. Switch 26 is operative to hold a closed circuit connectionbetween the terminals of the cable 50 and the leads 26 for heater III,-while the vapor tension Sylphon operates to open the circuit by forcingspring 16 and pin 66 upwardly against the adjustable tension of spring84, when the vapor tension in the Sylphon is high enough.

Capsule 12 of the Sylphon unit may be filled with water, alcohol, ether,various alcohol-water mixtures, or other organic or inorganic liquidshaving boiling points approximating the predetermined maximumtemperature to which the oil or other liquid in the tank is to beheated. For example, the capsule of a Sylphon employed with a heaterwhich is designed for keeping lubricating oil at a temperature of -140F. may be filled with ether.

The invention having been thus described, what is claimed as new is:

1. An electric immersion heater comprising a rigid bonded dielectricrefractory core, a helically coiled metal resistance wire surroundingand supported by said core, a thin bonded dielectric heat conductantrefractory coating embedding said wire coil, a flexible shock absorbentheat conductant dielectric filling surrounding the bonded refractorycoating of the coil, and a sealed tubular metal casing in contact withthe outer surface of the flexible dielectric filling and enclosing theelement, together with a tubular metal shield disposed in closely spacedrelation around the heater casing and having a plurality of radiallyarranged longitudinal grooves in the inner wall thereof together withside wall apertures for circulation of liquid to be heated in contactwith the heater casing.

2. Apparatus as defined in claim 1 in which said flexible dielectricfilling between the refractory coated coil and the outer metal casingcomprises finely ground ferrosilicon powder each grain of which iscoated with an oxide film.

JOHN D. MORGAN. PERCY B. LEVI'I 'I.

